Many kinds of substances have a deleterious effect on microorganisms. Antimicrobial substances have two kinds of activity, the one bacterial, germicidal or virucidal, concerned with the killing of microorganisms; and the other bacteriostatic or growth-inhibiting. Antimicrobial activity is a property of both inorganic and organic substances, and the exploitation of such activity is a matter of considerable practical importance in the development of antiseptics, sanitizers, germicides, bactericides, sporicides, virucides and disinfectants. A number of inorganic substances have antimicrobial activity because of the toxicity to microorganisms of the ions into which they dissociate; or because of their activity as oxidizing agents which brings about some degree of oxidation of cell substance. Among the inorganic substances which act as antimicrobial agents are salts. The degrees to which salts are effective as toxic agents are mainly dependent upon the degree of dissociation of the salt, the nature of the anion, and the valence and molecular weight of the metallic ion. In general, the bivalent cations are more toxic than the monovalent cations, and the salts of the heavier metals are more toxic than those of the lighter metals. The antimicrobial activity of the heavy metal salts is attributed to the affinity of the cations for protein material; when the constituent protein of the bacterial cell is precipitated as an insoluble proteinate, the cell dies. Other factors may also be involved, however.
The processes of effecting bacterial growth-inhibition or death are subject to a variety of influences. The most important of these influences is the concentration of the reacting substances, i.e., concentration of the biocidal substance and the numbers of bacteria present. The effective concentration of a biocidal substance is, in turn, dependent upon primarily two other factors: first, the presence of moisture, which makes possible ionization of the substance to form the biocidal agent and acts as a suspending medium in which there may be intimate contact between the biocidal agent and the microorganism. Second, presence of extraneous organic and/or other matter which will react with the agent prior to contact with an organism thereby rendering the agent ineffective.
Metal salts or metal complexes have been used as antimicrobial agents. Representative of prior art patents directed to the use of metal salts or metal chelates of inorganic or organic compounds as microbiocidal agents include: U.S. Pat. Nos. 871,392; 991,261; 1,679,919; 1,785,472; 2,208,253; 2,269,891; 2,456,727; 2,494,941; 2,878,155; 2,900,303; 2,901,393; 2,938,828; 3,076,834; 3,099,521; 3,206,398; 3,240,701; 3,262,846; 3,266,913; 3,681,492 and 3,782,471.
It has been established in antimicrobial activities that salts of heavy metals are rapidly precipitated by extraneous organic or other material and, therefore, while such salts may have an initial cell kill at an initial effective concentration, the effective concentration is rather quickly reduced by combination of the metal with such extraneous matter, thereby depleting the amount of toxic metal available for biocidal activity. Therefore, while inorganic salts offer in certain instances the property of aqueous solubility and, therefore, dissociation for availability as toxic metal agents, they may be rendered rather quickly ineffective such that prolonged or controlled destruction or inhibition of bacterial action is unavailable. On the other hand, metal salts or complexes of organic moieties such as organic acids or the like possess a degree of dissociation which is normally not as great in comparison to, for example, highly soluble inorganic salts. Therefore, whereas the metal organic salts or metal complexes may have a greater stability or kinetic inertness with respect to extraneous organic matter present in the environment of living cells, there is also generally a loss of toxic effect by reason of their higher stability.
Superimposed on this brief background discussion of antimicrobial activity is the relation of bacterial growth to the acidity or alkalinity of the media conducive to such growth. Concentrations of hydrogen ions compatible with growth are very low, generally, on the order of about 10.sup.-.sup. 4 to 10.sup.-.sup. 9 moles of hydrogen ions per liter. Almost all bacteria will grow at about pH 7.0 (1 .times. 10.sup.-.sup. 7 moles hydrogen ion per liter) but thrive best at optimum pH's which vary from species to species. The minimum and maximum limits between which viability is possible likewise vary widely with species. The activity, therefore, of an antimicrobial agent in the pH range of microbial viability is a very important consideration in that the activity determines biocidal efficacy.
In substance, heretofore in the prior art there appear to have been two extremes made available by antimicrobial metallic compositions. On the one hand, known metal compounds have a high degree of dissociation such that toxic metal ions are quickly and copiously made available by virtue of rapid dissociation with formation of ionized species. These species react so as to saturate all available ligands; and thereby are rendered inactivated as to cidal potential in a very narrow time frame, obviating residual killing power, thus rendering them relatively ineffective as biocidal agents over prolonged periods of time. Other known metal compounds are relatively stable and provide minimal amounts of ionized species over the normal physiological pH range, providing, therefore, minimal growth inhibitory or toxic potential, due to the very minimal degree of dissociation inherent in them.